Optical imaging has drawn great attention from various areas in the past decade, particularly industrial inspection and biomedical diagnosis. As the scientific research has gone deeper and deeper into the details of these areas, people have become more and more interested in dynamic behaviors, especially those involved in life science, e.g. hemokinesis, cytology and neurology, which provide information that may help provide a better understanding of the human body. In particular there is great interest in ultrafast dynamic diagnosis of disease and other tissue functionality. To visualize high-speed dynamic events, an optical imager is required to provide high sensitivity and high throughput. Modern photo-receivers can offer excellent sensitivity, e.g. widely-used CCD/CMOS cameras and photomultiplier tubes (PMT). Their imaging speed, on the other hand, is largely limited, which results in blurred images. Thus enhancing the speed of optical systems has become a hot topic in recent years. Typically, there are two main issues that have to be addressed for increasing the effective speed of an optical system: scanning speed and signal readout speed.
The conventional way to perform two-dimensional (2D) imaging is point-by-point scanning (i.e., raster-scanning) over the object through multi-dimensional translation stages. Mechanical inertia, unfortunately, limits the moving speed of these bulky stages, typically to several mm/s. For an example of such mechanical stages or actuators, see the LTA Precision Motorized Actuators, Series 300862, made by Newport Corporation. https://www.newport.com/Precision-Motorized-Actuators,-LTA-Series/300862/1003/info.aspx#tab_Specifications. It thus takes 10s of minutes or even hours to finish a 2D image scan.
The object or sample can be moved by mechanical stages through a beam or, rather than moving the sample, the more favored way is to scan the laser beam over the object by using high-speed scanning galvanometer mirrors (˜kHz) or acousto-optic deflectors (AOD, 10s of kHz), which can boost the 2D imaging frame rate to 100 Hz. See X. Chen, U. Leischner, Z. Varga, H. Jia, D. Deca, N. L. Rochefort, and A. Konnerth, “LOTOS-based two-photon calcium imaging of dendritic spines in vivo,” Nat. Protoc. 7(10), 1818-1829 (2012), which is incorporated herein by reference in its entirety. Although in the latter case, i.e., where the beam is moved, can indeed perform the video rate imaging, it is still far from the requirements for a 3D volumetric visualization of those highly dynamic objects.
In addition to the scanning speed, the readout time of the optical signal is another limitation on effectively boosting up the imaging speed, and it must be fast enough to acquire the fast scanning signal. Traditionally, the optical signal is read out through CCD/CMOS cameras, which however exhibit an unacceptable latency and result in a slow frame rate, typically 100 Hz, and hence a long read-out time. See, the Hyper Vision HPV-2 high speed video camera of Shimadzu Corporation of Kyoto Japan, http://www.shimadzu.com/an/test/hpv/hpv2_1.html, which is incorporated herein by reference in its entirety. To overcome those issues, the wavelength-swept source, e.g. Fourier-domain mode-locking (FDML) swept source, has recently been proposed to leverage those high-speed photodetectors (PDs) for fast imaging, and to enable the video-rate necessary for 2D imaging. See R. Huber, M. Wojtkowski, and J. G Fujimoto, “Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography,” Opt. Express 14, 3225-3237 (2006), which is incorporated herein by reference in its entirety. Unfortunately, the wavelength-swept source is a point source, and it is still necessary to perform point scanning via, e.g., galvanometer scanning, for 2D/3D imaging.
Optical time-stretch is an emerging powerful all-optical technique that can further enhance the wavelength-swept rate up to the MHz range. See, K. Goda, K. K. Tsia, and B. Jalali, “Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena,” Nature 458, 1145-1149 (2009), which is incorporated herein by reference in its entirety. However, it has the same issue as that of FDML, i.e., extra scanning is required. In particular, mechanical or electronic scanning units are needed to perform 2D imaging. More importantly, wavelength-swept sources such as those used in traditional flow cytometers, operate at a kHz level and exhibit a broad pulse waveform with limited instantaneous peak power, typically at the mW level. The nature of the time-stretch, i.e., high frequency chirping, also limits the instantaneous peak power. Thus it cannot be used for those applications requiring high peak power, for example multi-photon imaging. See, N. G Horton, K. Wang, D. Kobat, C. G Clark, F. W. Wise, C. B. Schaffer, and C. Xu, “In vivo three-photon microscopy of subcortical structures within an intact mouse brain,” Nature Photon., 7, 205-209 (2013), which is incorporated herein by reference in its entirety.